(1) Field of the Invention
The present invention relates to an anti-corrosion coating composition generally called a zinc-rich paint (hereinafter referred to as "ZRP"). More particularly, the invention relates to an anti-corrosion coating composition comprising a silicate as a binder and zinc powder as an anti-corrosive pigment, wherein ultrafine particulate aluminum oxide and/or ultrafine particulate titanium oxide is incorporated in a specific amount.
(2) Description of the Prior Art
Coating compositions comprising a silicate as a binder have been known in the art, and they are disclosed, for example, in the specification of U.S. Pat. Nos. 2,807,552, 2,914,413, 3,056,684, 3,239,521, 3,239,549, 3,241,990, 3,392,039, 3,454,410, 3,895,136 and 3,900,630.
Further, so-called inorganic zinc-rich paints (hereinafter referred to as "I-ZRP") formed by incorporating zinc powder into various alkali metal silicate binders as disclosed in the foregoing known literature references have been known from old. In the industry of manufacture of steel structures or the ship-building industry, there have been broadly adopted methods in which these paints are coated on surfaces of steel structures to control or prevent corrosion of steel.
Protective mechanisms, formulations, properties and uses of ZRP inclusive of I-ZRP are illustrated in detail in "Paint and Varnish Production", April pages 35-41, May pages 87-94, June pages 57-60 and July pages 75-76 (1964).
Further, I-ZRP comprising sodium silicate, potassium silicate or the like as a binder and zinc powder as an anti-corrosive pigment is disclosed in detail, for example, in the specifications of U.S. Pat. No. 2,509,875, U.S. Pat. No. 2,576,307, U.S. Pat. No. 3,372,038, U.S. Pat. No. 3,551,173 and British Pat. No. 1,189,436.
ZRP such as mentioned above prevents corrosion of steel coated therewith through the electrochemical activity of the large quantity of zinc powder which is incorporated therein. Based on the same technical concept, it has been tried to mix aluminum powder with zinc powder and use the resulting mixture. For example, there are known I-ZRP comprising a binder composed of potassium silicate or a mixture of potassium silicate and sodium silicate and an anti-corrosive pigment containing zinc powder having a particle size of 5 to 10.mu. and aluminum powder having a particle size of 10.mu. or less at a mixing weight ratio of 1:9 to 9:1 (see the specification of U.S. Pat. No. 3,423,229), a coating composition comprising a pigment mixture including 98 to 60% by weight of a zinc pigment and 2 to 40% by weight of an aluminum pigment and an appropriate binder (see Japanese Patent Publication No. 5571/63) and ZRP comprising a mixture containing aluminum powder and zinc powder at a weight ratio of 0.03 to 0.4 and a polystyrene resin. Further, as a heat-resistant paint for artificial satellites, rockets and the like, there is known a coating composition comprising an alkali metal silicate, aluminum oxide, potassium hydroxide, boric acid and zinc oxide.
In case of I-ZRP, more than 80% by weight of zinc powder is generally incorporated in the resulting coating and corrosion of steel is electrochemically controlled or prevented by the zinc powder.
However, as a result of the electrochemical reaction there are formed corrosion products of zinc, such as zinc hydroxide, zinc oxide and zinc carbonate (hereinafter, they will be inclusively called "white deposit"). Further, in general, as the amount of zinc powder in the coating is increased to enhance the electrochemical anti-corrosive action, the amount of white deposit generated is proportionally increased.
The electrochemical action of I-ZRP on steel is ordinarily expressed in terms of the anti-corrosive potential. From the theoretical and practical viewpoints, it is ordinarily considered that the anti-corrosive potential should be lower than -850 mV based on a copper electrode in a saturated copper sulfate solution.
In conventional I-ZRP, since it is a primary object to control corrosion of steel, a large quantity of a zinc powder is incorporated, and most conventional I-ZRP have an anti-corrosive potential of about -1020 mV. This potential is sufficient to prevent corrosion, but because of too low a potential, zinc is consumed excessively in an unnecessarily large amount. Accordingly, generation of white deposit is promoted and it is often observed that white deposit is densely formed on the entire surface of the coating of I-ZRP.
Deposition or accumulation of white deposit, of course, drastically degrades the appearance of the I-ZRP coating. When other paint is to be coated over the I-ZRP coating, it is necessary to remove such white deposit. If another paint is directly applied to the I-ZRP coating without removal of white deposit, adhesion is degraded and good results are not obtained. Moreover, even if another paint is applied after removal of white deposit, white dust gradually accumulates, and in extreme cases, peeling of the coating formed on the I-ZRP coating is readily caused.
In view of the foregoing, an ideal I-ZRP is desired to have such a characteristic property that corrosion of steel can be effectively controlled by incorporation of a large quantity of zinc powder and consumption of incorporated zinc powder can be maintained at a very low level, namely formation of white deposit can be completely prevented or can be controlled to a very low level. However, such an ideal I-ZRP has not yet been developed.